The present invention relates to a resistance measuring circuit including a measuring capacitor which, as controlled by a microcomputer.
In actual practice it is often necessary to measure the value of a resistance to determine a physical parameter which influences the value of the resistance. For temperature measuring, use is made typically of resistances whose values are a function of the temperature existing at the time. This is why to determine the temperature first a resistance value needs to be measured and the actual desired parameter, namely the temperature can then be determined from the measured value. A resistance measuring circuit of the aforementioned kind is described, for example, on page 2-186 of TEXAS INSTRUMENTS xe2x80x9cMSP430 Family Application Reports 2000xe2x80x9d (SLAA024), the basic circuit of which for this measuring is shown in FIG. 1. The complete measuring procedure is controlled by a microcomputer 10 which may be a TEXAS INSTRUMENTS type MSP430 microcomputer. The measuring circuit contains a measuring capacitor Cm which can be charged via a charging resistor RI to the supply voltage Vcc of the microcomputer 10. For this charging procedure the microcomputer 10 outputs at its terminal TP.3 the supply voltage Vcc whilst switching its terminals TP.0, TP.1 and TP.2 to a high impedance state. This results in a charging circuit which leads to ground Vss via the charging resistor RI and the measuring capacitor Cm. As soon as the measuring capacitor Cm has been charged to the supply voltage Vcc the microcomputer 10 switches the terminal TP.3 to a high impedance state whilst connecting the terminal TP.2 to ground Vss. This results in the measuring capacitor Cm being discharged to ground via the reference resistor Rref. On commencement of the discharge procedure, the microcomputer 10 starts a count which increments until the charging voltage of the measuring capacitor Cm at the input I 27 of the microcomputer 10 drops below a predefined threshold value. The count attained at this point in time is a measure of the time taken from the start of discharge to attaining the threshold value. Subsequent to this first discharge procedure, the measuring capacitor Cm is recharged to the supply voltage Vcc, resulting in the measuring capacitor grounding the terminal TP.1 so that the measuring capacitor Cm is discharged via the measuring resistor Rs1. The same as before, the time duration from the start of the discharge procedure up to attaining the threshold value is determined from the count. If, in addition, the value of the measuring resistor Rs2 is to be determined, a new charging and discharge cycle is implemented as described.
From the times measured and the value of the reference resistor Rref, the value of the measuring resistor Rs1 and correspondingly also, where necessary, the value of the measuring resistor Rs2 can be determined from the formula   Rs1  =      Rref    xc3x97                  t        Rs1                    t        ref            
How the necessary potentials are applied to the corresponding TP terminals in the microcomputer 10 relative to the terminal TP.0 thereof is evident from FIG. 1. In this arrangement, the necessary switches of the microcomputer 10 are formed by MOS transistors which have a relatively high resistance value in the ON condition which is usually termed the internal resistance Rdson. This internal resistance located in each case in the discharge cycle of the measuring capacitor Cm influences the measuring accuracy achievable with the measuring capacitor as shown in FIG. 1. It is particularly in applications demanding extremely high accuracy, for example in calorimeter temperature measurement, that the temperature-dependent synchronism error of each internal resistance has serious consequences. The discharge curve of the measuring capacitor Cm falls namely asymptotically to a value which is influenced by the internal resistance of the MOS transistor located in the discharge circuit at the time. The temperature-dependent synchronism errors of these internal resistances make it impossible to measure the resistance without additional complicated circuitry when very high accuracy is mandatory.
The resistance measuring circuit is charged in a first cycle to a predefined charging voltage and discharged via a reference resistor to a predefined discharge voltage before then being recharged in a second cycle to the charging voltage and discharged via the resistance to be measured to the discharge voltage, the microcomputer measuring in each cycle the time duration between the start of the discharge procedure and the point in time of attaining a predefined fixed value of the voltage between the charging voltage and the discharge voltage across the measuring capacitor and determining from the product of the reference resistance and the ratio of the time duration measured in the second and first cycle the resistance value to be measured.
The invention is thus based on the objective of providing a resistance measuring circuit of the aforementioned kind with the aid of which a very high measuring accuracy is achievable which is not influenced by the internal resistances of the analog switches used in controlling the discharge procedure.
To achieve this objective there is provided in the resistance measuring circuit, a closed loop for regulating the discharge voltage to a fixed predefined constant value.
By maintaining the discharge voltage constant with the aid of the closed loop, it is achieved that the measuring capacitor Cm discharges to a discharge voltage value which is not influenced by the internal resistance of a switch located in the discharge circuit. This results in the time needed to discharge the reference resistor and the resistance to be measured being exclusively a function of their resistance values so that the desired high accuracy is achievable.